1. Field of the Invention
The present invention relates to a projection encoder based on a triple-grating concept, and more particularly to an improved technique for enhancing the mechanical strength and reducing the manufacturing cost of a grating portion integrally formed in a semiconductor substrate.
2. Description of the Related Art
The present applicant has already proposed in JP-A 2000-321097 a projection encoder based on a triple-grating concept. The projection encoder of this type comprises an LED as a light source, a movable plate composed of a semiconductor substrate on which transmission gratings and photodetectors (photodiodes) are integrally formed at a constant pitch, and a reflecting grating plate (fixed plate) on which reflecting gratings are formed at a constant pitch, in which the movable plate is disposed between the LED and the reflecting grating plate.
In the projection encoder with this structure, the movable plate is integrated with the measurement object and is caused to move within a plane perpendicular to the optical axis of light emitted by the LED in the direction along which the light transmission gratings and photodiodes are arranged. Light emitted by the LED first strikes the back surface of the movable plate, passes through the transmission gratings on the movable plate, and illuminates the surface of the reflecting grating plate in a pattern of grating stripes. Reflecting gratings are also formed at a constant pitch on the reflecting grating plate, making it possible to reflect only those components of light striking the reflecting grating plate that irradiate the reflecting gratings. The reflecting grating image is caused to re-irradiate the movable plate, and is received by photodiodes shaped as vertical stripes and formed at a constant pitch and a constant width.
The photodiodes and transmission gratings shaped as vertical stripes on the movable grating plate function as separate gratings. Consequently, the amount of light received by a photodiode is converted, based on a triple-grating concept of three gratings including reflecting grating, to a sinusoid waveform in accordance with the relative movement of the reflecting grating plate and the movable grating plate. It is thus possible to obtain a pulse signal that corresponds to the relative speed of travel and is based on the photocurrent of the photodiodes, and to calculate the relative speed of travel on the basis of the pulse rate of the pulse signal.
In addition, arranging the photodiodes such that an A-phase signal and a B-phase signal differing in phase by 90 degrees can be obtained makes it possible to determine the movement direction of the movable grating plate on the basis of these two-phase signals.
Thus, the transmission gratings and photodetectors of the optical encoder disclosed in the above publication are fabricated using semiconductor manufacturing technology, so the precision of the relative positions thereof is high and they can be formed with excellent precision. In addition, the photodetectors formed in a pattern of vertical stripes at a constant pitch function as a grating, and the grating itself has a lens effect, so an optical lens system can be dispensed with and the device can be made smaller. Another feature is that the use of the triple-grating concept prevents resolution from being adversely affected by the width or variability of gaps between such reflecting gratings and light transmission gratings, so the adjustment operations needed to ensure the required mounting accuracy for the members provided with these gratings can be simplified, and the restrictions imposed on the mounting locations can be eased. In addition to this, the interval between the reflecting gratings and transmission gratings can be increased, making it possible to obtain benefits such as achieving better environmental resistance by, for example, housing the reflecting gratings in a protective casing.
However, a projection encoder with this structure has drawbacks such as the following. First, in a projection encoder with this structure, transmission gratings and photodetectors are integrally formed on a semiconductor substrate with the aid of semiconductor manufacturing technology. The drawback is that the semiconductor substrate portion on which the transmission gratings are formed has a structure in which a large number of transmission gratings is formed with a very small pitch in the thinly etched portions, and the mechanical strength is therefore very low.
A Deep-RIE device or other expensive devices must be used to form gratings with such a very small pitch by etching. For this reason, there is a drawback in that the manufacturing cost is higher.